Conventional working processes on a working object in a vertical direction, like drilling, reeling, tapping, or screwing, mostly are carried out using hand-held tools. However, those tools are often not held stably by the user, especially not exactly vertically. Any torque has to be borne by the hand of the user, so no uniform force will be applied in the working process, which may cause the tool to break or even injure the user. Therefore in a working process that demands a high degree of verticality, like drilling, a fixed machine is often used. If, however, the tool is installed in a fixed position, any change in the working position is cumbersome, causing inefficient working.
Therefore, recently control arms have been used for working processes. Such a control arm is fastened to a workbench and consists of two link rods forming an L-shaped arm. A system of lockable joints allows a wide range of angular positions of the two link rods to vary the working position. On the two link rods there is also a gas vessel, which allows the control arm to separate from the working object, when the working process is finished. So a working process in a vertical direction is possible for a wide range of positions. When, however, the two link rods and the gas vessel include a small angle, they interfere with each other. So there is a blind spot in the range of possible working positions, close to the fixed end of the two link rods. Then the link rods have to be extended outward to compensate for this, leading to a bigger space consumed by the control arm.
Furthermore, when a control arm with two link rods is employed to vary the working position, there is no way to damp the horizontal reacting force at high rotational velocities of the tool. So vibrations develop, leading to less stability and making control difficult.
Moreover, when the working position is changed, the two link rods have to be folded to a smaller angle. Then the gas vessel is compressed, and the gas vessel exerts a counteracting force, which the user has to overcome, thus making the change of the working position troublesome.